If I force myself to get up and MOVE, even when I don't feel like it, I find that after a few minutes, I FEEL like it, and end up getting a good workout!!!
When I work out, I give it ALL I've got, til I'm 'empty'. I am now learning that it's ok to just 'REST'. When your body has done all it can do, it's ok to rest for a day, or two. Then get right back at it! I've found that after a 2 day resting period, I am stronger and can do more!